In control systems for controlling the speed of D.C. motors, it has become customary to provide multiple speed settings for the motor and to provide a fixed time delay between shifting from one speed point to another. This allows time for the motor to accelerate and develop sufficient counter E.M.F. to thereby limit the current to an acceptable maximum value to prevent destruction of the components in the circuit and the motor.
One type of control system that has been utilized for this purpose includes a plurality of acceleration resistors that are placed in series with the armature of the DC motor. A plurality of normally open contacts are respectively placed in parallel with each of the acceleration resistors so that the resistors can be bypassed sequentially by closure of the respective contacts.
The respective contacts are usually enclosed by separate circuits having contactor coils located therein and a time module is also placed in series with each contactor coil. Thus, energization of a contactor coil circuit by moving a master switch from a previous control step to the next control step will delay the energization of the contactor coil for a predetermined fixed time period, called an anticipation delay. After the setting of the time delay has expired, the timer module completes the circuit to the contactor coil to allow closure of the associated set of contacts to bypass the associated resistor and increase the speed of the motor.
One type of time module that can be utilized for this purpose consists of a resistor-capacitor exponential curve and a programmable unijunction transistor voltage comparator (hereinafter called P.U.T.). Upon expiration of the anticipated delay period, the P.U.T. provides a current pulse to the gate of a gating element such as a silicon controlled rectifier (hereinafter called S.C.R.). If the master switch for the DC motor controller is then moved or advanced to the next speed setting or control step, the acceleration contactor coil energization is again delayed by the next timer module for its predetermined fixed time.
The design criteria utilized for establishing the resistance, as well as the time delay, is usually calculated based upon a maximum load and speed for the load, as well as the intermediate motor loads and speeds.
In many areas of operation, the anticipated delay is disadvantageous when the operator is performing functions at less than maximum load and unnecessary if the motor has completed acceleration before advancing to the next step of acceleration. For example, if the operator is raising and lowering less than maximum loads, it is desirable to advance the speed setting to the next typical speed without awaiting the time delay incorporated into the above type of circuit. In other instances, the operator may be switching between control speed steps for the motor in rapid succession to produce a "jogging" sequence. When a jogging sequence is utilized in the existing control circuit, the operator must anticipate the time delay incorporated into the system when the speed is set to the next higher speed setting because the timer is not activated until the master switch is advanced to the next higher speed.
One type of control system that, to some degree, alleviates the above problem is disclosed in co-pending application Ser. No. 040,806, filed May 21, 1979, for a Pre-Step Timer Circuit for DC Motor Control. The solution disclosed in this application consists of placing a sensing resistor module between the line side of an accelerator contactor coil of one speed control step to the load side of the accelerating coil contacting coil and to the input of a timer module in series therewith in a subsequent speed control step. Thus, when the first speed control step is activated, the resistor module will produce a feed forward path to the next timer to allow a small amount of current to flow through the resistor module to operate the next timer in a subsequent control step which permits the accelerating contactor coil of the subsequent control step to be actuated immediately when the master switch is advanced.